python Function instance: remove duplicate code

Author: RonnyPfannschmidt

src/_pytest/python.py

@@ -1596,11 +1596,8 @@ def instance(self):
         try:
             return self._instance
         except AttributeError:
-            if isinstance(self.parent, Class):
-                # Each Function gets a fresh class instance.
-                self._instance = self._getinstance()
-            else:
-                self._instance = None
+            # Each Function gets a fresh class instance.
+            self._instance = self._getinstance()
         return self._instance
 
     def _getinstance(self):